1. Field of the Invention
The present invention relates generally to a surface acoustic wave (SAW) filter utilizing longitudinal double-mode resonances, and more specifically to such a SAW resonator filter which is capable of effectively suppressing a spurious response at a lower side of the filter's passband.
2. Description of Related Art
It is well known in the art that SAW resonator filters have been widely used in telecommunication equipment wherein narrow passband and low insertion loss are required.
Before turning to the present invention it is deemed preferable to discuss, with reference to FIGS. 1-4, a conventional SAW resonator bandpass filter using longitudinal double-mode resonances.
FIG. 1 schematically illustrates a relevant SAW resonator filter and three longitudinal resonance modes excited in the above filter in parts (a) and (b), respectively.
As shown in FIG. 1, a SAW resonator filter, depicted by numeral 8, includes a piezoelectric substrate 10 which carries thereon a pair of so-called apodized interdigital transducers (IDTs) 12a-12b and two grating reflectors 14a and 14b. These elements 12a-12b and 14a-14b are arranged in a longitudinal direction of the substrate 10 (viz., an acoustic wave propagating direction).
As shown, each of the IDTs 12a-12b is provided with a plurality of overlapped electrode pairs which are arranged in a symmetrical manner at both sides of a transversal center line of the element.
Although not clearly illustrated in the figure, each of the IDTs 12a-12b has few hundreds of electrode pairs, while each of the reflectors 14a-14b has about one hundred electrodes in parallel, merely byway of example.
The SAW resonator filter shown in FIG. 1 receives an input signal at the IDT 12a via an input terminal 16 and outputs a filtered signal from the IDT 12b via an output terminal 18.
When an input signal is applied to the SAW filter 8, three longitudinal resonance modes RM1-RM3 are excited which are schematically shown in part (b) of FIG. 1. The first resonance mode RM1 is established due to multireflections of acoustic waves between the IDTs 12a-12b. The second resonance mode RM2 is induced within the IDTs 12a and 12b. On the other hand, the third resonance mode RM3 is excited due to multiple reflections of acoustic waves between the reflectors 14a-14b.
The existence of the above mentioned longitudinal resonance modes has been discussed in a paper entitled "Composite Longitudinal Mode Resonator Type SAW Filters" by Yasushi YAMAMOTO, the Japanese Electronics, Information and Telecommunications Association, A Vol. J76-A No. 2, pages 219-226, published February 1993.
If the device shown in FIG. 1 is to operate as a two-pole bandpass filter, the longitudinal resonance modes RM1 and RM2 are utilized by appropriately determining distances D1 and D2 shown in FIG. 1.
FIG. 2 is a sketch schematically showing frequency-loss characteristics of the device shown in FIG. 1. Three peaks, depicted by P1, P2 and P3, correspond to the aforesaid three longitudinal mode resonances RM1, RM2, and RM3, respectively.
The peak P3 appearing at the low side outside of a passband generally defined by the peaks P1 and P2, is a spurious response and thus should be suppressed in order to obtain a sharp lower cut-off response. However, with the device shown in FIG. 1, it has been difficult to effectively suppress the spurious response. The reason for this will be discussed with reference to FIG. 3.
FIG. 3 shows, in part (a), the device already shown in FIG. 1. FIG. 3 further shows three standing waves SW1-SW3 in part (b), and curves 20a and 20b in part (c). These curves 20a and 20b schematically show "receive signal distribution" excited in the IDTs 12a and 12b. It should be noted that the curves 20a and 20b resemble the envelopes of the electrode overlap portions of the IDTs 12a and 12b, respectively.
The standing wave SW1, which is induced by the longitudinal resonance mode RM1, distributes over a cavity length La defined by the outer end of each of the IDTs 12a and 12b. Secondly, the standing wave SW2 is developed when the longitudinal resonance mode RM2 is excited. This standing wave SW2 distributes over a cavity length Lb defined by the inner end of each of the reflectors 14a and 14b. Lastly, the third standing wave SW3 is caused by multiple reflections of acoustic waves between the two reflectors 14a and 14b (viz., the resonance mode RM3) and distributes over a cavity length Lc specified by portions within the reflectors 14a and 14b.
As shown in part (c) of FIG. 3, each maximum of the curves 20a and 20b is shifted inward by n distance "m" as compared with the zero crossing point of the standing wave SW3. Accordingly, the above mentioned "receive signal distribution" is unable to effectively cancel the standing wave SW3. This is the reason why the undesirable spurious peak P3 is not sufficiently suppressed. It should be noted that this mechanism or principles was discovered by the inventors.